Small scale biogas digesters are used increasingly to supply biogas to households worldwide in. Biogas thus produced is used for different applications, amongst others cooking. In many small-scale digesters configurations it is quite common that no further treatment of the biogas is applied before it is burnt. This is in particular the case in developing countries, where the need for purifying gases before burning them conflicts with the perceived associated costs thereof.
Hydrogen Sulfide (H2S) is present in varying concentrations in biogas. H2S has negative effects on the durability of the equipment, because when burnt it may be converted into sulfur oxides, which are highly corrosive, in particular in combination with water, thus resulting in high material replacement costs. Moreover, the presence of sulfur oxides due to combustion of H2S can have detrimental consequences for the people exposed to the flue gases. Furthermore, H2S is very poisonous by itself and exposure must be avoided at all times Therefore removal of H2S is essential for an optimal functioning of the biogas system, as well as for the health of the users of these systems and their environment.
Biogas is produced during anaerobic digestion of organic materials such as manure, sewage sludge, organic fractions of household and industrial waste and energy crops. It is typically produced at atmospheric pressure. Biogas can be used as a renewable energy source, for example as fuel for vehicles or as a substitute for natural gas. Additional advantages are a lower release of methane into the atmosphere (methane is a known greenhouse gas) compared to traditional manure management and landfills, as well as the simultaneous production of a high quality digestate for applications as fertilizer.
The exact composition of the biogas is amongst others dependent on the type of material used in the anaerobic digestion. Typically it contains 50-70 vol % methane, 30-50 vol % CO2 and 0-4000 ppm H2S. These high amounts of CO2 and H2S in biogas result in a relatively low energy content per volume, and therefore needs to be treated prior to use as energy source. Raw biogas, i.e. biogas as directly obtained after the anaerobic digestion, can be cleaned of unwanted substances such as particles, water, H2S and CO2 in a process called upgrading. The resulting upgraded biogas is much higher in methane content than the raw biogas.
Several countries have defined standards with which the upgraded biogas must comply before grid injection or utilization as vehicle fuel. In 2010, the European Commission mandated CEN (M/475) to set European standards for upgraded biogas requirements. This stresses the importance of the biogas upgrading process.
H2S removal from gas streams is commonly carried out using a regenerative absorption solution. One of the typical classes of such regenerative systems comprises an agent that can oxidize H2S into elemental sulfur (S0). In this reaction the oxidizing agent is reduced. A commonly used oxidizing agent is for instance Fe(III)NTA, a complex of nitrilotriacetic acid, N(CH2COOH)3, NTA, a chelating agent and Fe(III). Normally an aqueous solution of oxidizing agent is used. In a subsequent step the reduced oxidizing agent is regenerated by oxidizing it back to its original state, for instance by reacting it with oxygen. Such a process is well known, see for instance EP-A-0 215 505. Current systems are for large scale application that have separate vessels in which the absorption of H2S and the regeneration of the solvent takes place. Although these systems are commercially interesting systems, they cannot be economically used for small-scale household biogas applications due to relatively high capital and operational cost at the small scale level. Moreover, the complexity level exceeds the applicability demands for small scale.
It is an object of the present invention to provide a cost-effective, simple and robust, regenerative system for effective removal of H2S from biogas obtained from small scale biogas digesters.
It was found that this and other objectives may be reached by carrying out the H2S oxidation step and the regeneration of the oxidizing agent in the same vessel, and optionally at the same time, viz. without requiring a separate regeneration vessel, or regeneration phase. The oxidation and regeneration step can be carried out simultaneously or sequentially.